Fix enum validation when default attribute exists

This patch allows an undefined instance to validate correctly with the
“enum” attribute if a “default” attribute is also provided that
validates with the “enum” attribute.

An undefined instance with with a “required” attribute of `true` will
still fail to validate (from the “required” attribute).
Merge pull request #33 from tJener/fix/undefined-validate-enum-with-default

Fix enum validation when default attribute exists